In the electronics industry there is a steady trend towards manufacturing microprocessors of increasingly high speed and large information storage capacity. This requires the individual electrical devices such as transistors, etc. within the microprocessors to be fabricated at an increasingly small scale. The metallic electrical interconnects between the devices also need to be miniaturized. As device and interconnect dimensions approach one-half to one-quarter of a micron, the choice of interconnect metal becomes critical. The large current densities resulting from small interconnect cross sectional areas can lead to major problems such as electromigration, stress migration, and voiding where the metal lines become fractured or otherwise physically degraded under operating conditions, a major drawback with aluminum alloys. Metal interconnects also need to provide the lowest electrical resistance path possible since resistance-capacitance delays become a dominant factor in circuit performance at sub half micron levels. The aluminum that is widely used in present day interconnect manufacturing is reasonably conductive (2.7 microohm cm), but needs to be alloyed with 0.5 to 4.0% Cu to minimize the electromigration tendencies of the pure metal. Tungsten, also widely used, is electromigration resistant but is of higher resistivity (5.4 microohm cm). Considering these facts, copper should be an excellent interconnect metal as it is both highly conductive (1.7 microohm cm) and electromigration resistant.
Metallic interconnects are typically horizontal lines (runners) or plugs (vias) that wire together devices in microprocessors. At feature sizes of &gt;1 micron these metallic components can be built into the microcircuits by PVD (Physical Vapor Deposition) techniques such as sputtering or evaporation. In essence PVD consists of condensing a metal vapor onto a surface or into a hole or channel of a circuit where an electrical connection needs to be formed. Since this is a non-selective metallization, either a postdeposition clean-up (i.e. etch-back) or a predeposition masking of the substrate (i.e. the lift-off technique) is required in order to prepare individual discreet metal components. However, the severe surface topographies presented by sub-micron features preclude the effective use of PVD since this "line of sight" technique cannot provide a uniform conformal coating on such high aspect ratio highly convoluted surfaces. Specific examples of these shortcomings include the phenomena of geometrical shadowing and poor step coverage.
A superior process for producing these microscopic metal features is CVD (Chemical Vapor Deposition). In this technique a volatile metal-organic compound in the gas phase is contacted with areas of a circuit where growth of a metal film (i.e. interconnect) is required. A surface catalyzed chemical reaction then occurs which leads to deposition of the desired metal. Since this is a chemical reaction, there is potential for it to provide surface selective metallization. That is, CVD metal deposition can be made to occur at only specific locations compared to the non-selective PVD processes. Also, since the metal film steadily grows on the desired surface it is of a uniform thickness and highly conformal even to severe geometries. In this respect CVD is naturally suited to fabricating submicron high aspect ratio features.
An example of selective CVD metallization that is currently commercially practiced is the deposition of tungsten onto a silicon surface using tungsten hexafluoride as the volatile organometallic precursor (see T. Ohba, et al., "Tungsten and Other Advanced Metals for VLSI/ULSI Applications V," Ed. by S. S. Wong and S. Furukawa, MRS, Pittsburgh, Pa., 273 (1990)). The chemistry that drives this process can be divided into two steps. Initially the WF.sub.6 reacts with the elemental silicon surface to yield tungsten metal and volatile silicon hexafluoride. Hydrogen gas is then added to the system which reduces further WF.sub.6 at the freshly formed metal surface thereby yielding additional tungsten and HF gas. Although this system currently enjoys widespread use as the only "selective" CVD metallization process that is widely commercially available, loss of selectivity can be observed and is commonly ascribed to the corrosive nature of HF. T. Ohba, et al., Tech. Dig. IEDM, 213 (1987) teach the use of silane as a reducing agent for WF.sub.6 to achieve higher deposition rates while avoiding the production of HF gas.
Desirable selectivities for a copper CVD process include deposition onto conducting metallic or metallic like surfaces such as tungsten, tantalum or titanium nitride versus insulating surfaces such as silicon oxide. These metallic surfaces provide a diffusion barrier between the CVD copper and the underlying silicon substrate that the device is grown upon.
Copper films have previously been prepared via CVD using various copper precursors. Most of these compounds will only deposit copper metal at temperatures higher than 200.degree. C. with no significant selectivity between substrates such as diffusion barrier surfaces vs. silicon oxide. The best known and most frequently used CVD copper precursor is copper.sup.+2 bis(hexafluoroacetylacetonate). This highly fluorinated organometallic precursor is significantly more volatile than its parent unfluorinated complex copper.sup.+2 bis(acetylacetonate) and its ease of vaporization readily lends this compound towards CVD processes. The use of this compound as a general precursor for CVD copper metallization was first described by R. L. Van Hemert et al. J. Electrochem. Soc. (112), 1123 (1965) and by R. H. Moshier et al. U.S. Pat. No. 3,356,527. More recently Reisman, et al., J. Electrochemical Soc., Vol. 136, No. 11, November 1989 and A. E. Kaloyeros et al., Journal of Electronic Materials. Vol. 19, No. 3, 271 (1990) in two independent studies have also evaluated the use of this compound as a copper precursor for electronics applications. In these studies copper films were formed by contacting vapors of copper.sup.+2 (hfac).sub.2, mixed with either an inert gas (argon) or with hydrogen and contacting the mixture with a heated substrate surface. In the case of using hydrogen the copper.sup.+2 atom in the precursor complex is formally reduced to copper metal while the hfac.sup.-1 ligand becomes protonated to yield a neutral volatile compound. In the case of using an inert gas the copper.sup.+2 (hfac).sub.2 is simply pyrolyzed to give copper metal and fragments of the hfac ligand.
Pure copper is reported for the hydrogen reduction but oxygen and carbon are found in the films obtained by pyrolysis. However, the lowest deposition temperatures for either process is 250.degree. C. and no strong selectivities towards metallic vs. silicon oxide surfaces are reported. Copper films have also been prepared from copper.sup.+2 (hfac).sub.2 by plasma enhanced deposition, C. Oehr, H. Suhr, Appl. Phy. A. (45) 151-154 (1988), laser photothermal decomposition, F. A. Houle., C. R. Jones., T. Baum., C. Pico., C. A. Korae; Appl. Phys. Lett. (46) 204-206 (1985), and photochemical decomposition of copper.sup.+2 (hfac).sub.2 ethanol adducts, F. A. Houle., R. J. Hilson; T. H. Baum., J. Vac. Sci. Technol. A (4), 2452-2458 (1986). Some of these methods yield fluorine contaminated films and none are reported to yield selective depositions. Similar hydrogen reduction of volatile copper compounds has also been demonstrated by Charles et al. U.S. Pat. No. 3,594,216 using copper.sup.+2 .beta.-ketoimine complexes at 400.degree. C. to deposit copper metal films onto glass or quartz substrates. No mention of selectivity is made. G. S. Girolami, et al., Chem. Mater. (1) 8-10 (1989) reported using copper.sup.+1 t-butoxide to yield copper films by CVD at 400.degree. C., but the resultant films were impure in that they contained 5% oxygen.
The only CVD precursors known to deposit pure copper metal films below 200.degree. C. are the copper.sup.+1 cyclopentadienyl phosphine compounds described by Beech et al., Chem. Mater. (2) 216-219 (1990), but these are also not reported to be strongly selective towards metallic or metallic like surfaces vs. silicon oxide or similar insulating dielectrics. An additional problem that this particular class of compounds faces for electronics applications is their potential to contaminate microcircuits with phosphorus, an element that is extensively used as a silicon dopant.